This invention is in the field of three dimensional object sensing apparatuses and methods and more particularly in the field of apparatuses and methods for sensing fingerprints.
Fingerprint sensing devices typically consist of a glass plate or prism upon which the finger of the subject is pressed, an illuminating light source which directs the light through the prism to the contact zone between the finger and the glass plate or prism, an image producing lens and an image capturing device such as a charge couple device (CCD). Most of these known devices function on the basic principle that if the angle of incidence of the illuminating light with the contact zone between the finger of the subject and the glass plate or prism, is adjusted to within a certain range which is dependent on the frequency of the light used, the illuminating light will be internally reflected from the contact zone at fingerprint valley points where there is no contact between the finger and the glass surface, and will not be internally reflected from the ridge points where the finger contacts the glass surface. The reflected light is focused into an image by an image producing lens and the image is transformed into an electrical signal by a CCD or other sensing device.
These known devices have inherent problems and limitations relating to the accuracy of the valley and ridge information defining the subject fingerprint. Known methods require the use of an image producing lens and require an optical sensor such as a CCD to receive the image and convert it to an electrical signal. Improvements are needed in the means and method of illuminating the contact zone between the finger and glass surface, the physical geometry of the sensing unit and the means and method of receiving and digitizing the fingerprint information.
An object of the present invention is to provide a simpler, more accurate and more reliable method and apparatus for extracting and digitizing a fingerprint for imaging and identification purposes. It is a further object of the present invention to provide a method and apparatus for utilizing an array of light-emitting diodes (LED""s) for emitting point source radiation as the source of illuminating radiation. It is a further object of the present invention to provide a fingerprint sensing device that is durable, inexpensive and simpler to manufacture and maintain. It is a further objective of the present invention to provide an apparatus and method for the direct digitization of a fingerprint without the use of an image producing lens and optical image sensor.
Preferred embodiments of the apparatus of the present invention comprise a prism, a pixilated source of radiation, and a photoelectric sensor. For certain preferred embodiments, the prism is wedge-shaped, but it can be of any shape that provides for differentiation between fingerprint ridges and valleys. The finger of the subject is pressed to the prism top surface. For certain preferred embodiments, an LED array, which is used as the pixilated radiation source, lines the prism bottom surface and the photoelectric sensor lines the prism end surface. However, the pixilated source of radiation can be any array of point source radiation emitters which can be focused respectively on selected points of illumination on the prism top surface.
For preferred embodiments using an LED array, the LED""s of the LED array are arranged in a grid network with the spacing between the LED""s being selected to provide for the desired accuracy of the fingerprint extraction and digitization. Each of the LED""s emits a sub-beam into the prism bottom surface when activated and each of these LED""s corresponds to a point of illumination on the prism top surface. The wedge angle between the prism top surface and the prism bottom surface determines the angle of incidence for each of the sub-beams. The angle of incidence of the sub-beams is selected so that the points where the fingerprint ridges of the subject contact the prism top surface will not internally reflect the LED radiation, and the points corresponding to the fingerprint valleys of the subject where the finger of the subject does not contact the prism top surface will reflect the radiation.
Each LED of the LED array is activated in a desired sequence thereby transmitting the light into the bottom surface of the prism and illuminating a corresponding point on the top contact surface of the prism. However, the illuminating radiation is not confined to the corresponding point of illumination. Accordingly the photoelectric sensor will receive reflected radiation for each LED. If the LED corresponds to a fingerprint ridge point, the amount of illuminating radiation reflected to a photoelectric cell will be measurably less than that reflected if the LED corresponds to a fingerprint valley point. The difference can be increased by incorporating a minute lens at each LED point. The lenses are fixed to the LED array or the prism or are interposed between the LED array and the prism as a lens wafer. The lenses can also be incorporated as an integral part of the prism. These lenses provide for the focusing of the radiation transmitted from each LED on the corresponding point of illumination on the top contact surface of the prism. For these embodiments, the non-reflection of the illuminating radiation will be near total from fingerprint ridge points and the reflection of the illuminating radiation will be near total for the fingerprint valley points, thereby enhancing differentiation between ridge points and valley points. The number of pixels and thus the accuracy of the resolution of the fingerprint extraction, is determined by the density of the LED""s.
For some embodiments, each sub-beam is reflected to a unique point on the photoelectric sensor. The photoelectric sensor may have a grid of receptors, each receptor sensing the reception of a sub-beam. The reception or non-reception of a sub-beam indicates a fingerprint valley or ridge at the corresponding point of illumination. For other embodiments, the sub-beams may be emitted sequentially in a burst and therefore sensed sequentially by a single-receptor photoelectric sensor. Each sub-beam received in its assigned time slot indicates a fingerprint valley at its corresponding point of illumination and each sub-beam not received in its assigned time slot indicates a fingerprint ridge at its corresponding point of illumination. Either version provides for a very accurate digitization of the fingerprint, which can then be used for imaging or identification purposes. This provides for the direct digitization of the subject fingerprint without the need for an image producing lens or an optical image sensing device.
The photoelectric sensor may have a filter or may be tuned to detect only the frequency of radiation emitted by the LED so as to prevent interference from radiation from other sources. Alternatively, filters or shields may be installed on all exposed exterior surfaces of the prism to prevent admission of radiation to the prism from any source other than the surface emitting laser.
For any of the foregoing embodiments, the fingerprint apparatus of the present invention may be connected to an external computer to process the data, or may be equipped with its own computer circuit, particularly for remote applications.
For any of the foregoing embodiments, an additional lens or lens arrangement, a sensor lens, may be included between the prism and the photoelectric sensor, to provide for focusing the reflected radiation for sensing. This allows the use of a smaller photoelectric sensor or a smaller grid of receptors for the photoelectric sensor. The sensor lens may be affixed to or incorporated into the prism end surface, or may be interposed between the prism and the photoelectric sensor.